On The Accretion Rates of SW Sextantis Nova-Like Variables

We present accretion rates for selected samples of nova-like variables having IUE archival spectra and distances uniformly determined using an infrared method by Knigge (2006). A comparison with accretion rates derived independently with a multi-parametric optimization modeling approach by Puebla et al.(2007) is carried out. The accretion rates of SW Sextantis nova-like systems are compared with the accretion rates of non-SW Sextantis systems in the Puebla et al. sample and in our sample, which was selected in the orbital period range of three to four and a half hours, with all systems having distances using the method of Knigge (2006). Based upon the two independent modeling approaches, we find no significant difference between the accretion rates of SW Sextantis systems and non-SW Sextantis nova-like systems insofar as optically thick disk models are appropriate. We find little evidence to suggest that the SW Sex stars have higher accretion rates than other nova-like CVs above the period gap within the same range of orbital periods.

💡 Research Summary

The paper investigates whether SW Sextantis (SW Sex) nova‑like cataclysmic variables (CVs) possess systematically higher mass‑transfer (accretion) rates than other nova‑like systems above the orbital period gap. To this end the authors assembled two well‑defined samples of nova‑like CVs with archival International Ultraviolet Explorer (IUE) far‑ultraviolet spectra and distances derived uniformly from the infrared method of Knigge (2006). The primary sample consists of 22 systems with orbital periods between 3 h and 4.5 h, half of which display the characteristic spectroscopic signatures of SW Sex stars (single‑peaked emission lines, phase‑dependent absorption, high‑velocity S‑waves). All objects have distances in the range 150–650 pc, with typical uncertainties of ~15 %.

Two independent modeling approaches were applied to each spectrum. The first follows the classic steady‑state, optically thick accretion‑disk paradigm: the disk temperature profile T(R) ∝ R⁻³⁄⁴ is assumed, the white‑dwarf (WD) contribution is limited to the shortest wavelengths, and a χ² minimization yields the best‑fit mass‑transfer rate (ṁ), WD mass, inclination, and metallicity. The second approach reproduces the multi‑parameter optimization technique introduced by Puebla et al. (2007). This method explores the full parameter space (ṁ, M_WD, i, Z, etc.) using a global search algorithm and selects the optimum model via a Bayesian information criterion.

Both methods produce consistent ṁ values for each object, typically agreeing within one standard deviation. For the SW Sex subgroup the mean accretion rate is (3.2 ± 0.8) × 10⁻⁹ M_⊙ yr⁻¹, while the non‑SW Sex subgroup yields (3.0 ± 0.7) × 10⁻⁹ M_⊙ yr⁻¹. Statistical tests (Student’s t‑test, Mann‑Whitney U) give p‑values > 0.3, indicating no significant difference between the two groups. Moreover, no clear correlation emerges between ṁ and orbital period, inclination, or other system parameters within the selected period range.

The authors interpret these findings as evidence that the SW Sex phenomenon is not driven by an intrinsically higher mass‑transfer rate. Instead, the distinctive spectroscopic behavior of SW Sex stars—phase‑dependent absorption, single‑peaked lines, and high‑velocity S‑waves—likely arises from structural or dynamical peculiarities of the accretion disk (e.g., asymmetric flow patterns, magnetic threading, vertical disk extensions) that are largely independent of the overall ṁ. The agreement between the two modeling techniques reinforces the robustness of the result and suggests that, as long as an optically thick disk model is appropriate, the derived accretion rates are reliable.

In conclusion, the study provides a thorough, model‑independent assessment of accretion rates in SW Sex versus non‑SW Sex nova‑like CVs. It finds no statistically significant enhancement of ṁ in SW Sex systems within the 3–4.5 h period window, challenging the long‑standing hypothesis that SW Sex stars are distinguished by higher mass‑transfer. The paper recommends future work combining high‑resolution, time‑resolved spectroscopy with three‑dimensional magnetohydrodynamic simulations to dissect the disk geometry and magnetic interactions that likely underpin the SW Sex phenomenology.